1. Field of the Invention
The present invention refers to a metallic implant to be applied to a human or animal bone, the implant having a porous surface to come in contact with the bone and to intergrow therewith. Implants of this kind are used as prostheses in medicine, more specifically in orthopedics, for replacing broken or diseased bone, and in dentistry, for building artificial teeth. The implant must be made of a non-corrosive material and must be compatible with the surrounding tissue, meaning that it should not produce an immunologic reaction effecting rejection by the body. The invention also refers to a method of making the metallic implant.
In the following the term "surface" or "contact surface" shall be used to refer to the implant surface intended to be brought into contact with the bone to form an "interface" therewith, at which interface the bone will intergrow with the implant, forming a bond strong enough to resist all of the mechanical forces it will be subjected to while in use. This surface may be an inner or an outer surface, for example a cylindrical surface, depending upon the specific application and corresponding geometry of the implant.
2. Description of the Prior Art
Implants are known to be used in orthopedics and dentistry in various sizes and shapes. Among the larger ones are the prostheses used to replace the spherical element in the upper joint of the hip bone or femur, among the smaller ones are pins screwed into jaw bones for building artificial teeth. An implant of this kind is frequently made of one of the metals or elements titanium, zirconium, niobium or tantalum, or of a tissue-compatible alloy having one of the aforementioned elements as its main component. A common problem with such implants consists in the necessity of implementing the measures required for the bone substance to establish a speedy and lasting bond or connection with the contact surface of the implant. Other terms used in the art for this process include "implant anchoring" and "osseointegration".
With reference to bone implants, the following statements were made in the Abstract of a Study by the Work Committee for Implants of the German Society for Materials Testing, made public in Berlin on Nov. 17, 1987:
"Combined tension and histomorphometric tests have shown that smooth contact surfaces of titanium implants do not provide adequate interfaces that would resist tension forces. Contact surface roughnesses of more than 20 .mu.m are required if a tension-resistant bone implant bond or connection is to be built. Uniform geometrical surface patterns as well as non-uniform porous designs are capable of improving the tension strength of the bone-titanium interface or bond, especially if additional sandblasting is applied."
It is customary nowadays to coat the contact surface of an implant with a titanium plasma coat, or to produce a surface roughness thereon by sandblasting or by threading said contact surface. Of common knowledge are both the drawbacks of the surfaces so treated and the fact that contact surface roughness of this kind is required for achieving adequate adhesion between bone and implant. Such drawbacks essentially consist in the fact, that the mechanically brittle plasma layer has a tendency to break or peel off, and in that the surfaces roughened by sandblasting become contaminated by the blasted grains, most often corundum. Attempts made to subsequently to clean the contact surface by means of a pickling or corrosive solution such as hydrofluoric acid plus nitric acid (HF+HNO.sub.3) resulted in a substantially less perfect intergrowth of bone substance and implant, and weaker anchoring of the implant in the bone through its contact surface.
Terms used in this specification are generally based on definitions in METALS HANDBOOK, desk edition, by Boyer and Gall, American Society for Metals, Metals Park, Ohio, 1985. According to this text (pages 27.20-27.25), roughness, one of the four elements of surface texture, is the most commonly used surface parameter. Further, surface measuring devices generally indicate the roughness, but do not indicate the physical character of the surface and in effect, several surfaces can be quite different in appearance and still yield similar roughness values. Among roughness parameters are the maximum peak-to-valley height (R.sub.t), which can be determined by a surface measuring device, and peak count (P.sub.c), which is the number of peak/valley pairs per linear unit of surface and the reciprocal of which is roughness spacing (RS). Thus, R.sub.t concerns perpendicular distances, while RS concerns horizontal distances and together these parameters provide a more accurate picture of the surface texture than either parameter alone.